Field
The described embodiments relate generally to determining one or more physical parameters associated with a sample by iteratively converging measurements of a physical phenomenon associated with the sample with a forward model that predicts the physical phenomenon based on the one or more physical parameters.
Related Art
Many non-invasive characterization techniques are available for determining one or more physical parameters of a sample. For example, magnetic properties can be studied using magnetic resonance or MR (which is often referred to as ‘nuclear magnetic resonance’ or NMR), a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation. Moreover, density variations and short or long-range periodic structures in solid or rigid materials can be studied using characterization techniques such as x-ray imaging, x-ray diffraction, computed tomography, neutron diffraction or electron microscopy, in which electromagnetic waves or energetic particles having small de Broglie wavelengths are absorbed or scattered by the sample. Furthermore, density variations and motion in soft materials or fluids can be studied using ultrasound imaging, in which ultrasonic waves are transmitted and reflected in the sample.
In each of these characterization techniques, one or more external excitation (such as a flux of particles or incident radiation, static or time-varying scalar fields, and/or static or time-varying vector fields) are applied to the sample, and a resulting response of the sample, in the form a physical phenomenon, is measured. As an example, in MR magnetic nuclear spins may be partially aligned (or polarized) in an applied external DC magnetic field. These nuclear spins may precess or rotate around the direction of the external magnetic field at an angular frequency (which is sometimes referred to as the ‘Larmor frequency’) given by the product of a gyromagnetic ratio of a type of nuclei and the magnitude or strength of the external magnetic field. By applying a perturbation to the polarized nuclear spins, such as one or more radio-frequency (RF) pulses (and, more generally, electro-magnetic pulses) having pulse widths corresponding to the angular frequency and at a right-angle or perpendicular to the direction of the external magnetic field, the polarization of the nuclear spins can be transiently changed. The resulting dynamic response of the nuclear spins (such as the time-varying total magnetization) can provide information about the physical and material properties of a sample, such as one or more physical parameters associated with the sample.
In general, each of the characterization techniques may allow one or more physical parameters to be determined in small volumes or voxels in a sample, which can be represented using a tensor. Using magnetic resonance imaging (MRI) as an example, the dependence of the angular frequency of precession of nuclear spins (such as protons or the isotope 1H) on the magnitude of the external magnetic field can be used to determine images of three-dimensional (3D) or anatomical structure and/or the chemical composition of different materials or types of tissue. In particular, by applying a non-uniform or spatially varying magnetic field to a sample or a patient, the resulting variation in the angular frequency of precession of 1H spins is typically used to spatially localize the measured dynamic response of the 1H spins to voxels, which can be used to generate images, such as of the internal anatomy of a patient.
However, the characterization of the physical properties of a sample is often time-consuming, complicated and expensive. For example, acquiring MR images in MRI with high-spatial resolution (i.e., small voxels sizes) often involves a large number of measurements (which are sometimes referred to as ‘scans’) to be performed for time durations that are longer than the relaxation times of the 1H spins in different types of tissue in a patient. Moreover, in order to achieve high-spatial resolution, a large homogenous external magnetic field is usually used during MRI. The external magnetic field is typically generated using a superconducting magnetic having a toroidal shape with a narrow bore, which can feel confining to many patients. Furthermore, Fourier transform techniques may be used to facilitate image reconstruction, at the cost of constraints on the RF pulse sequences and, thus, the scan time.
The combination of long scan times and, in the case of MRI, the confining environment of the magnet bore can degrade the user experience. In addition, long scan times reduce throughput, thereby increasing the cost of performing the characterization.